A conventional motor controller is described hereinafter with reference to FIG. 17 and FIG. 18. Driving a synchronous motor involves (a) detecting a position of magnetic pole of the motor, and (b) controlling a current or a voltage applied to windings of the motor responding to a magnetic pole signal (CS signal) indicating the position of magnetic pole of the motor.
FIG. 17 shows rectangular wave drive of a three-phase motor. Based on a logic of magnetic-pole signals CS1, CS2, and CS3 of the three-phase, a rectangular wave drive of 120 degrees is applied to phases U, V and W of the motor. Power is usually supplied during this 120 degrees period. FIG. 18 shows a sine wave drive of the three-phase motor. A sine wave drive applied to the phases U, V and W of the motor using (a) a change point of the CS signals' logic and (b) a positional information from the change point produced by a position detector of high resolution, such as an encoder, separately mounted. In this sine-wave drive, power is supplied during 180 degrees period.
The sine wave drive is desirable because it can drive a motor efficiently with less vibrations. However, as described above, CS signals simply throw a motor in rectangular wave drive, thus a motor controller needs a positional detector such as an encoder for obtaining positional information in order to throw the motor in sine wave drive. The positional detector should be mounted separately, which is unfavorable to the motor controller in view of the cost and size. Even a motor controller including an encoder is obliged to drive the motor with a rectangular wave at initial starting because an absolute position cannot be detected during a period from the starting to a first change of a CS signal. In this period, the motor cannot be driven with a sine wave, and the rectangular wave drive is only a choice.
Japanese Patent Application Non-examined Publication No. H10-201284 discloses that a constant speed drive of a motor allows sine wave drive by dividing intervals between change points of the logic of the CS signal. The intervals are measured by a timer, and the intervals are divided by the measured values. However, this method cannot deal with a great change in a motor speed or a transient period, so that the sine wave cannot be kept going.